It frequently is required in industry to separate a slurry containing different grain sizes in a wide range e.g. between 5 and 150 .mu.m into two phases so that the grains predominantly smaller than a given grain size, e.g. 40 .mu.m pass into the first phase, and those predominantly larger, than the given grain size pass into the second phase. This requirement exists for example in the alumina industry, where the slurry containing the precipitated alumina hydrate is transfered to a hydro-separator (primary thickener) which divides it into two phases according to the requirements. The hydro-separator is a tank with vertical axis having usually a conical bottom and being provided with overflow on the upper rim as well as with a slurry outlet stub on the lower end and a coaxical inlet pipe along its axis the lower end of which is submerged deeply in the slurry in the tank. In a steady-state operation mode the tank is full of slurry, the continuously supplied fresh slurry to be sized passes through the inlet pipe into the tank, flowing downwards, but since the outflow rate of the slurry outlet stub is less than the slurry-feed rate of the inlet pipe, a certain part of the slurry passes upwards to the overflow. As a result, three well distinguishable zones are formed in the tank. These are: so-called settling zone on the bottom, in which the slurry current moves slowly and steadily downwards; the so-called sizing zone above the level of the lower orifice of the inlet pipe in which the slurry current flows slowly and steadily upwards; and a so-called transition zone between the two zones, in which a certain part of the slurry moving downwards from the inlet pipe passes upwards with a 180.degree. directional change.
It is known that in given liquid the final falling velocity of the grains consisting of given solid matter is dependent on the size and shape of the grains. Depending upon whether the final falling velocity is lower or higher than the velocity of the upward flowing liquid, the grains move upward or fall. The upwardly flowing grains pass into the overflow, the downwardly flowing grains into the lower part of the tank.
The hydro-separator works the more effectively the less is the number of the undesirable marginal grains passing into the lower part of the tank.
The drawback of known equipment for this purpose is that the direction of the flow in the transition zone is not definite; there the flow conditions are complex and dead spots develop. The dead spots do not participate in the sizing and a certain part of the slurry passes directly into the settling zone carrying away the solid grains independently from their sizes, the greater part after more or less swirling passes into the sizing zone where the sizing takes place virtually exclusively.